1. Field of the Invention
The present invention relates to an injection mechanism for an injection molding machine.
2. Description of the Related Art
An injection molding mechanism for an injection molding machine is generally constructed in such a manner that a movable plate rotatably carrying an injection screw is slidably attached onto a guide rod arranged parallel to the rotation axis of the injection screw, and that the movable plate is driven by a drive section along the guide rod so as to axially move the injection screw, whereby an injection (or advancing) step, a metering (or backing) step, and so on, are performed.
For example, Japanese Unexamined Patent Publication (Kokai) No. 8-192447 (JP8-192447A) discloses an injection mechanism for an injection molding machine, which includes a front plate carrying an injection cylinder, a rear plate oppositely facing and spaced from the front plate and tied to the front plate through a plurality of tie rods, and a movable plate arranged between the front and rear plates and movable along the tie rods. The movable plate carries an injection screw in a rotatable but axially immovable manner, and, on the other hand, is slidably attached to the tie rods, acting as guide rods, through sliding bearing bushes provided in the movable plate. In this condition, the rotation axis of the injection screw is located generally parallel to the guiding axes of the tie rods. Also, a drive section for moving the movable plate along the tie rods is provided as an electric motor and a feed screw unit operated by the electric motor.
In the conventional injection mechanism as described above, the movable plate slides on the tie rods (or guide rods) through a sliding bearing mechanism in the rotation axis direction of the injection screw, in both of metering and injection steps. Therefore, a clearance is inevitably created between the inner circumferential surface of the sliding bearing bush and the outer circumferential surface of the tie rod, which results in a play between the sliding bearing bush and the tie rod. Therefore, there is a tendency that an eccentricity or an axis inclination is caused between the sliding bearing bush and the tie rod, when the movable plate moves forward or backward, which may generate a so-called stick-slip motion or increase a sliding frictional resistance, thereby an injection pressure in the injection step or a back pressure in the metering step may fluctuate, and consequently an injection process may become unstable. In particular, in the above-described arrangement in which the drive section for the movable plate includes the feed screw unit, there is a case where a reaction force against the rotation of a screw shaft in the feed screw unit is transmitted to the tie rods (or guide rods) through a ball nut in the feed screw unit, mounted on the rear plate and supporting the screw shaft. In this case, the eccentric mutual engagements between the tie rods and the sliding bearing bushes are likely to become significant, and, as a result, an injection operation and/or a metering operation may become further unstable.